This invention relates generally to mechanically frothed plastisols, and in particular to mechanically frothed, PVC dispersions.
Although the use of chemical blowing agents is well established for preparing plastisol foams, mechanical frothing techniques are also known. Mechanical frothing involves the use of a continuous roamer or mixing head that mechanically aerates or froths a plastisol prior to delivery. A metering device may control the flow rate of pressurized gas into the metered plastisol stream before entry of the stream into the mixing head, which typically includes an intermeshing stator and rotor. The foam may be dispensed continuously or intermittently.
In intermittently dispensed foam applications, foam may be delivered from the mixing head through a dispensing nozzle by means of a pump and a positive displacement metering device that actuates a dispensing valve. Pressures in excess of about 150 psi, even up to about 200 to 400 psi, are customarily used to open the dispensing valve. An approximately fifty percent loss of foam may typically occur between the mixing head and dispensing nozzle such that even when a high percent foam on an order of about 45% is formed in the mixing head, the percent foam dispensed may be no more than about 15 to 20%.
As illustrated by U.S. Pat. No. 4,423,161 to Cobb, Jr. et al, it is known to maintain a substantially constant composition of a blowing agent in a vapor space above a foamable liquid under a vapor pressure at least equal to the vapor pressure of the foamable liquid, so as to dispense foam of constant foam-volume ratio. Although applicability of such a method to plastisols and the use of a gaseous agent is suggested in this patent, no mention is made of use of a mechanical frothing technique, nor is information given as to the percent foam dispensed or its structural uniformity.
In the production of certain products, it is desirable to dispense a high percent plastisol foam and for the foam to be homogeneous, using an automated process for forming and dispensing the foam. However, to achieve a high percent plastisol foam with a chemical blowing agent is costly; moreover, uniformity of the foam would be difficult to obtain. Also, in intermittently dispensing a fluid such as a foam, drip from a dispensing head may deleteriously affect the shot size and quality of product. In the case of a low viscosity fluid, drip becomes more difficult to prevent.
Accordingly, there is a need for an automated process for production and delivery of a high percent plastisol foam, and in particular a homogeneous, high percent plastisol foam, using mechanical frothing. To achieve such a foam, it will be advantageous to determine the causes of loss of percent foam and to overcome or avoid the causes. Moreover, for safety reasons, it will be beneficial to maintain the processing pressure below about 100 psi. Furthermore, when it is important for the shot delivered to be constant, an improved technique for preventing drip from a dispensing head would be advantageous. Beneficially, such a technique would reduce waste.